EP0098224B1 - Bone prosthesis and its manufacture - Google Patents

Abstract

An osteal prosthesis having an outer surface splined in a direction generally parallel to its mean axis is capable of being secured in the bone without cement, the bone tissue developing so as to at least partly occupy the interior or hollow between the splines. The cross-sectional profile of the splines is such that it imparts a bone tissue retaining capability, and the longitudinal interior surface or hollow of the splines, which are non-cylindrical, has a shape which tapers from the end of the prosthesis (lower end) which is the most deeply embedded in the bone to the other end (upper end), the prosthesis being thus capable of being extracted from the bone tissue by a tractive movement.

Description

The present invention relates to the field of bone prostheses. It relates to a new prosthesis and its obtaining.

The general technique of implantation of metal parts intended to replace or consolidate bones or bone joints is already known. Of course, the shape of the implant must be adapted to the bone part concerned. This is the case, for example, of prostheses intended for the hip or knee joints.

Metallic implants were first used by fixing them in living bone with a cement, in particular with a cement based on acrylic resin. This technique of fixing with a cement leads to numerous drawbacks, among which one can report a defect in the holding of the prosthesis due to the risks of fractionation of the cement, as well as intolerance by the body and the bone tissue of the foreign body that constitutes the cement.

This is why we proposed prostheses whose installation did not require the use of cement. To this end, the surface of the implant has been made rough or porous, so as to allow the development of bone tissue on the surface of the prosthesis itself, and thus to anchor it in the bone. The disadvantages of the cement fixing technique are almost entirely eliminated by this technique, but it can still be improved. By way of illustration, it can be estimated that the use of cement provides, in the case of hip prostheses, more than 25% of failures, due to the loosening of the implant in the bone. This percentage of failures is significantly reduced when the implant is given a porous or rough surface, but in at least 10% of cases, for very varied reasons, it is necessary to intervene again in order to replace the prosthesis . But, in this case, the prosthesis cannot simply be extracted because, due to the extremely resistant anchoring effect, provided by the development of bone tissue, it is necessary, to replace the prosthesis, to carry out a delicate intervention with cutting of the bone to take out the prosthesis.

As prior documents illustrating the state of the art, mention may be made of DE-C 837 294, FR-A 2194123, US-A 3905047 and FR-A 2 356 465. The teaching of these various patents can be briefly summarized as follows.

DE-C 837 294 relates to a prosthesis whose axis (or tail) has an outer surface grooved in a general direction parallel to the mean axis. No precise information is given on the grooves, the shape, number and importance of which may be any. Likewise, their profile can be at an acute angle or rounded. In fact, these grooves cannot ensure a sufficient retention effect of the bone tissue and, moreover, the rectilinear longitudinal profile of these grooves does not allow extraction of the prosthesis tail under satisfactory conditions.

FR-A 2194123 proposes a prosthesis intended to be fixed without cement, and comprising a porous external surface. Once in place, such a prosthesis cannot be removed. In addition, the conditions for obtaining such a prosthesis lead to a risk of corrosion of the surface. It is also made more fragile which, under the effect of high stresses, can cause areas of weakness and ultimately lead to fracture of the prosthesis.

US Pat. No. 3,905,047 relates to a prosthesis comprising surface irregularities. Once in place, such a prosthesis cannot be extracted by a simple tensile force, due to the development of bone tissue, which constitutes one of the characteristics claimed by the patent.

FR-A 2356465 relates in particular to a prosthesis comprising on its outer surface a coating consisting of beads or spherules. These are distributed at random to ensure a solid anchoring effect. Such a prosthesis cannot, without special intervention, be extracted in the event of an accident.

Many prostheses are also known, the installation of which requires the use of a cement. The tails of some of these prostheses are shaped by presenting grooves or recessed longitudinal parts, essentially in order to lighten the prosthesis and ensure good insertion into the cement. This is the case, in particular, of the prosthesis described in document GB-A 2069340, which describes a prosthesis in which the tail of the prosthesis is fixed in the bone, by means of cement, so as not to ability to move longitudinally or in rotation relative to the bone in which the prosthesis is inserted.

It is therefore found that, at present, to the knowledge of the applicant, there are no bone prostheses which can be placed without cement, capable of being retained effectively in the bone tissue, after development of that -this, and at the same time to be extracted, if necessary, without being obliged to practice a delicate intervention. The subject of the invention is a new prosthesis which simultaneously fulfills all of these requirements.

The subject of the invention is also the obtaining of such a prosthesis, under industrial conditions making it possible to end up with a product having a mechanical resistance much greater than that of the prostheses currently known. In particular, the production process does not cause any weakening of the surface of the prosthesis.

• - la surface intérieure des cannelures présente un profil en section transversale comportant une partie rétrécie (10 fig.4) qui s'épanouit vers le fond (9) de la cannelure, ce profil assurant la rétention du tissu osseux développé à l'intérieur de la cannelure;
• - la surface intérieure longitudinale des cannelures, non cylindrique, présente une forme allant en s'amincissant depuis l'extrémité dite basse (4) de la prothèse, c'est à dire celle qui s'enfonce le plus profondément dans l'os jusqu'à l'autre extrémité (3) dite haute, la prothèse pouvant ainsi être extraite du tissu osseux, par un mouvement de traction.

The subject of the invention is a bone prosthesis intended to be fixed without cement, and having on its part intended to be driven into the bone an outer surface grooved in a direction generally parallel to its mean axis, said prosthesis being characterized in that:

• - the inner surface of the grooves has a cross-sectional profile comprising a narrowed part (10 fig. 4) which opens out towards the bottom (9) of the groove, this profile ensuring the re retention of bone tissue developed inside the groove;
• - the longitudinal inner surface of the grooves, not cylindrical, has a shape which tapers from the so-called low end (4) of the prosthesis, that is to say that which sinks most deeply into the bone up to 'at the other end (3) called high, the prosthesis can thus be extracted from the bone tissue, by a pulling movement.

The prosthesis according to the invention constitutes an improvement to the object of DE-C 837 294, by providing, in combination, the desired anchoring effect for the bone tissue and the faculty of extraction. The transverse profile of the grooves must provide a high power of retention of the bone tissue. Contrary to the teaching of DE-C 837 294, this profile must therefore not be open, but have a narrowed part, so that the developing bone tissue can be trapped at the bottom of the groove. For this purpose, the transverse profile can be curvilinear or circular, or a combination of these two types of profiles, the arcs being able to be connected by rectilinear parts. In combination with this transverse profile, the grooves made on the external surface of the prosthesis according to the invention have, in the longitudinal direction, a slight taper between the end driven into the bone and that which, if necessary, must be gripped. for extraction. The advantageous results of the invention are obtained with taperings the value of which can be relatively low. A slope of 1% to 1% o has been found to be suitable.

The invention is applicable to all kinds of metal implants which can serve as prostheses, or used for the treatment of bones. Particularly interesting results are obtained in the case of hip or knee prostheses, and in general with all prostheses comprising a rod-shaped part. Such a rod can be straight or have a certain curvature, which allows it to better match the shape of the bone in which it is intended to be anchored. Likewise, the outer surface of the rod does not necessarily take the form of a regular cylinder. It is even many advantageous cases, in particular to satisfy the requirements of the resistance of the materials, to conform the rod by giving it a gradually increased thickness, at least in its upper part. Some of the grooves extend over the entire length of the rod, and some others can, if they start from the top, reach an edge of the rod without going to its end.

In another aspect, the invention relates to a process for obtaining a fluted prosthesis part by the general lost wax casting technique, in which a wax model is produced corresponding to the part to be manufactured, performs a refractory molding on said model to form a mold, and liquid metal is poured into this mold to obtain the desired part, said method being characterized in that, before refractory molding, the grooves of the model are placed in wax, rods whose transverse profile corresponds to that of the grooves of the final part and which have a slight taper from one end to the other, so as to obtain, after pouring the metal into the refractory mold, a part with grooves whose non-cylindrical longitudinal inner surface has a shape which tapers from the low end to the high end.

In the case, for example, of a prosthesis piece comprising a tail, the wax tail is first produced to the dimensions of the piece to be reproduced, by providing concave grooves parallel to the periphery of the tail. 'axis. Rods, for example made of polystyrene or wax, having the length of the grooves to be produced, and having a slight taper from one end to the other, are manufactured. The profile of the rods, in cross section, corresponds to that of the grooves of the final part. These rods are assembled, in particular by gluing, in the concave grooves of the tail. We thus produced a wax model of the part to be manufactured. We then proceed in a known manner, to the refractory coating of this wax model, to produce a refractory mold and the final part is obtained by pouring the liquid metal into such a mold, from which any wax has been previously removed.

In another aspect, which corresponds to a preferred embodiment, the invention relates to a process for obtaining a piece of fluted prosthesis, characterized in that it is carried out by the general known technique of wax casting lost, a smooth metal blank having a slight excess thickness with respect to the transverse dimensions of the part to be manufactured, in that said blank is treated successively by hot forging then by cold forging, until the final dimensions of the part are obtained desired, and in that one realizes thereon, by electro-machining, longitudinal grooves having the desired transverse profile and whose longitudinal inner surface, non-cylindrical, has a shape that tapers from the lower end to the high end.

In such a process, first of all, by the lost wax casting technique, a part model, for example of a tail, is produced, the outer surface of which is smooth and which has a slight excess thickness relative to the transverse dimensions of the part. to obtain. As a concrete example, for a shank of about 15 mm in diameter, an additional thickness of 1 mm per side is suitable. The rough foundry blanks are then taken up by hot forging to obtain an intermediate part, still slightly extra thickness (in the example above it can be around 0.50 mm). The final calibration giving the final dimensions is carried out by cold forging. From this forged shank, with a smooth outer surface, the grooves are made by removing metal, using the electro-machining technique. For this purpose, a tool is used which is moved in the longitudinal direction of the tail, to dig the grooves there. We can operate by successive passes, using tools with different profiles. In a variant, which is preferred from the economic point of view, a single tool is used in one pass, in order to achieve the desired transverse profile of grooves, namely allowing an optimal retention effect of the bone tissue during the development thereof.

• Fig. 1 est une vue de face de la prothèse;
• Fig. 2 est une coupe faite selon la ligne II-II de la figure 1 ;
• Fig. 3 est une vue de l'intérieur d'une cannelure faite suivant la flèche III de la figure 1;
• Fig. 4 montre à une échelle agrandie, un profil de cannelure;
• Fig. 5 est une figure analogue à celle de la figure 4, montrant une variante de profil;
• Fig. 6 est un schéma illustrant un procédé de fabrication de la prothèse selon l'invention;
• Figs. 7 et 8 sont des schémas illustrant un autre procédé préféré de fabrication de la prothèse selon l'invention.

The process which has just been described is the most interesting in practice. In fact, starting from the blank coming from the foundry, the stages of hot and cold forging very significantly improve the mechanical characteristics of the part. The electro-machining technique fully respects the mechanical properties, and can be implemented on conventional equipment, which need only include a tool that meets the requirements of the invention. The invention will now be illustrated without being in any way limited, with the aid of the description below, which relates to a prosthesis intended to be inserted into the femur (hip prosthesis), with reference to the appended drawings in which:

• Fig. 1 is a front view of the prosthesis;
• Fig. 2 is a section taken on the line II-II in FIG. 1;
• Fig. 3 is a view of the interior of a groove made according to arrow III in FIG. 1;
• Fig. 4 shows, on an enlarged scale, a groove profile;
• Fig. 5 is a figure similar to that of FIG. 4, showing a variant profile;
• Fig. 6 is a diagram illustrating a method of manufacturing the prosthesis according to the invention;
• Figs. 7 and 8 are diagrams illustrating another preferred method of manufacturing the prosthesis according to the invention.

Figures 1 and 2 show, schematically, a prosthesis part intended to be inserted into the femur. In general, it consists of two parts, a tail (1) and a part (2) which, for convenience, will be called "collar". In the example chosen, the tail (1) is approximately 200 mm long, and its transverse dimension, approximately, is 15 mm. Figure 2 shows more clearly, in section, the slightly elliptical shape of the tail (1). The end (4) goes deepest into the femur bone. The end (3) is shaped as shown in Figure 1, so as to provide maximum mechanical strength. The tail (1) also has recesses, two of which have been shown (5,6).

According to the invention, the tail (1) has grooves (7). Some of the grooves (7), in particular the middle part, extend over the entire length of the tail (1). Other grooves, in particular in the vicinity of the upper part (3) or of the recesses (5,6) do not extend to the end (4), but stop at the corresponding external edge of the tail. We also see in Figures 1 and 2, that a part (8) of the neck, located in extension of the tail (1) is also grooved. This part (8) is limited to one face of the neck (2). The essential feature of the invention is that the grooves (7,8) have a transverse profile capable of ensuring effective retention of the bone tissue when it develops after insertion of the prosthesis. Figures 4 and 5 show examples of suitable profiles. In fig.4, we see two adjacent grooves (7a, 7b), delimiting between them a bottom (9) or inner groove surface. It can be seen that, in FIG. 4, the interior surface (9) is delimited by a curve which narrows as indicated in (10), before opening out to form the groove proper. It is the conformation of the interior surface which ensures the anchoring effect and the retention capacity of the bone tissue. It goes without saying that the drawing in FIG. 4 is greatly enlarged (magnification approximately 10 times).

Fig. 5 shows an alternative embodiment with two adjacent grooves (7c, 7d). The groove bottom (11) has two bosses (12, 13) approaching the upper part. In this case also, the desired retention effect is ensured.

According to another fundamental characteristic of the invention, which combines with that concerning the transverse profile of the groove, the longitudinal profile of the latter is not cylindrical, but on the contrary forms a draft. As illustrated in Figure 3, the inner surface of a groove (7) tapers from the end (4) pressed deepest into the bone, to the end (3) near the collar (2). It is clear that the representation of Figure 3 is schematic. In practice, with the previous tail dimensions (namely: length 200 mm, diameter 15 mm) the transverse dimension of a groove in the vicinity of the end (4) is for example 1.5 mm, and it is 1.3 mm near the end (3). This flared profile from top to bottom, allows extraction of the prosthesis if necessary.

For extraction, one can provide on the neck (2), a pair of notches (14) (see fig. 1) allowing the surgeon to insert a tool for the extraction of the prosthesis.

Schematically illustrated in FIG. 6, a process for obtaining a prosthesis tail, of the kind shown in FIGS. 1 to 5. This process uses the general technique of lost wax casting. FIG. 6 represents the tail (15) in wax which is produced according to the dimensions of the part to be produced. It has on its entire periphery concave grooves (16) parallel to the general axis of the tail (15). Rods (17) are also produced in polystyrene or in wax, having a transverse profile similar to the grooves of FIGS. 4 and 5, and a longitudinal profile in relief. Thus, their diameter at the end (18) is greater than that of the end (19). These chopsticks are individually put in place by gluing in the concave grooves (16) of the wax tail. The procedure is then carried out in a known manner, by coating the wax model obtained in accordance with the representation of FIG. 6 with refractory material. From the refractory mold, liquid metal can be poured to produce the final prosthesis part.

It is preferred to manufacture the prosthesis according to the invention by a process in which, first of all, by the lost wax casting technique, a tail model is produced, the outer surface of which is smooth, and the transverse dimensions of which are slightly enlarged by compared to those of the final part to be obtained. With the figures indicated above (length of the tail of 200 mm and diameter of 15 mm), the excess thickness of the wax model can be approximately 1 mm on each face, or 2 mm in all. A metallic tail blank having such an extra thickness is then produced. The blank is then taken up by hot forging to obtain an intermediate part having, relative to the final part, a reduced excess thickness, for example of approximately 0.50 mm per face. The final calibration is obtained by cold forging, which considerably improves the mechanical characteristics of the prosthesis. The final grooved shape of the prosthesis according to the invention is obtained by removing metal by an electro-machining technique. This has been illustrated schematically in Figures 7 and 8. Figure 7 shows in section the tail 21 of the prosthesis to be manufactured. The latter is held horizontally in a fixing tool 22 which only reveals the upper half 21a of the tail 21. The longitudinal grooves or grooves 23 are obtained by actuation of a pair of electrodes 24, 25 The active end of each electrode covers half of the upper part 21a a of the tail, so that this upper part 21a is completely covered by the electrodes, as shown in FIG. 7. For EDM or electro- machining, each electrode 24, 25 is approached from the part 21 a (arrows 26) in a convergent movement, after which the two electrodes withdraw (arrows 27). The piece (tail 21) is then turned over and the same operation is carried out on the other half 21 b.

Fig.8 shows in an enlarged manner the constitution of an electrode element 24, 25. We see in the drawing two adjacent grooves 28, 29 separated by a groove 30. In the example chosen, the groove 30 has a profile resulting from the combination of circular arcs 31 and 32, of opposite curvature. The bottom of the groove is formed by a flat 33.

The element 34 of the electrode, which is made of copper, is designed to perform the machining of the profile 31, 32, 33 and, for this purpose, it has arcuate parts 35, 36 and an end part 37. internal channels are provided in the electrode element 34 to allow the passage of an electrolyte, as shown by the arrows in FIG. 8. It can thus be seen that the element 34 has a main channel 39, two lateral passages 40, an axial channel 41, and two other lateral passages 42. Thanks to the shape of the electrode element 34 and the circulation of the electrolyte (which may be brine), the metal of the tail is removed. prosthesis according to the desired profile for the groove 30 between the grooves 28, 29 both in the transverse direction and in the longitudinal direction.

It goes without saying that the manufacturing methods described above are only simple examples. Those skilled in the art will understand that variants can be provided, in particular to adapt the manufacturing technique to the nature of the material of the prosthesis.

Thus the blank or blank of the prosthesis can also be obtained by hot forging from a cylindrical bar of appropriate dimensions. Hot forging requires a number of passes which depends on the nature of the material. At the end of the forging, a part is obtained having high mechanical characteristics.

The above techniques are well suited for stainless steels such as chrome-cobalt steels, for example that of grade HS21. Of course, other materials can be used, in particular other metal alloys.

Claims (9)

1. Bone prosthesis intended to be attached without cement, and possessing, on its portion which is intended to be inserted into the bone, an external surface channelled in a direction generally parallel to its median axis, the said prosthesis being characterized in that:

the internal surface of the channels has a transverse sectional profile incorporating a narrowed portion (10 Figure 4) which broadens out towards the bottom (9) of the channel, this profile providing for the retention of the boney tissue which has developed inside the channel; and

the shape of the non-cylindrical longitudinal internal surface of the channels becomes thinner from the so-called bottom end (4) of the prosthesis, that is to say the end which is inserted more deeply into the bone, to the other, so-called top end (3), the prosthesis thereby being able to be extracted from the boney tissue by a pulling movement.

2. Prosthesis according to Claim 1, characterized in that the transverse sectional profile of the channels is curvilinear or circular or a combination of these two types of profile, the arcs being able to be connected by rectilinear portions.

3. Prosthesis according to one of Claims 1 and 2, characterized in that the transverse dimension of the interior of a channel at the top end (3) is less than that of the bottom end (4).

4. Prosthesis according to Claim 3, characterized in that the ratio between the transverse dimensions of the top end and the bottom end is between 0.99 and 0.099.

5. Process for producing an item of channelled prosthesis, according to any one of Claims 1 to 4, by the general technique of cire perdue casting in which a wax model is made corresponding to the item to be manufactured, refractory moulding is performed on the said model to form a mould and liquid metal is cast in this mould to produce the desired item, the said process being characterized in that, before the refractory moulding, rods (17) made, for example, of wax or polystyrene and having the transverse profile which corresponds to that of the channels in the final item, and which possess slight conicity from one end to the other, are placed in the channels (16) of the wax model so as to produce, after casting the metal in the refractory mould, an item with channels, the shape of whose non-cylindrical longitudinal internal surface becomes thinner from the bottom end to the top end.

6. Process for the production of an item of channelled prosthesis according to any one of Claims 1 to 4, characterized in that, by the known general technique of cire perdue casting, a smooth metal blank is made which possesses a slight overmeasure relative to the transverse dimensions of the item to be manufactured, in that the said blank is treated successively by hot forging and then by cold forging until the final dimensions of the desired item are obtained, and in that longitudinal channels (28, 29) having the desired transverse profile (30), and the shape of whose non-cylindrical longitudinal internal surface becomes thinner from the bottom end to the top end, are produced in the item by electroerosion.

7. Process according to Claim 6, characterized in that, during the electroerosion, at least one pass is made using two electrodes (24, 25) working in a dipping convergent movement (26) and having a profile corresponding to that of the channels.

8. Process according to one of Claims 6 and 7, characterized in that the procedure is carried out successively on the halves (21 a, 21 b) of the prosthesis shank (21), which is maintained horizontally in a tool (22).

9. Process for producing a prosthesis according to any one of Claims 1 to 4, by hot forging from a cylindrical bar.

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